A conventional differential amplifying circuit comprises a differential pair of transistors for amplifying input signals, and a transistor for fixing output signals of the differential amplifier, wherein collectors of the differential pair of the transistors are commonly connected through resistances to a power supply and directly to output terminals, emitters thereof are commonly connected through a resistance to the ground, and bases thereof are connected to input terminals, respectively, while the output signal fixing transistor is connected, in collector to collector and emitter to emitter, to one of the differential pair of the transistors, and a base thereof is connected to a control unit.
In operation, input signals which are out of phase are supplied to the input terminals connected to the bases of the differential pair of the transistors, so that the input signals are differentially amplified therein to provide amplified output signals which are out of phase between the output terminals connected to the collectors of the differential pair of the transistors. Where extraordinary input signals are applied to the input terminals, a high level control signal is supplied from the control unit to the base of the output signal fixing transistor. Consequently, the output signal fixing transistor is turned on, so that one of the differential pair of the transistors is connected through its own collector and the output signal fixing transistor to the ground. Thus, the output signals which are out of phase are fixed to avoid the transmission of the extraordinary input signals to the output terminals.
However, the conventional differential amplifying circuit has a disadvantage in that an offset occurs between the output signals due to imbalance of loads induced between the differential pair of the transistors, because a parasitic capacitance produced between the collector and the emitter of the output signal fixing transistor is loaded to one of the pair transistors, to which the output signal fixing transistor is connected, where the output signal fixing transistor is turned off in the ordinary state. The conventional differential amplifying circuit has a further disadvantage in that wave forms of the output signals are different from each other, since one of the output signals is delayed in rising-up and falling-down times relative to the other due to the parasitic capacitance. Consequently, an inverted signal which is one of the output signals does not become equal in absolute value to a positive-phase-sequence signal which is the remaining one of the output signals.